Control device for load-independent flow regulation

ABSTRACT

A control apparatus for load-independent flow regulation, with at least one control unit having a slide valve, a pressure difference balance, conduits leading to a pump and to a consumer and a conduit communicating the pressure in the consumer unit to the pressure difference balance, is disclosed. This last conduit has first and second sections. The first section passes through the slide valve and, when the slide valve is moved out of the neutral position, opens into an annular groove provided in the slide valve bore. The second section extends from this annular groove and has located therein an operating piston which in one position connects a chamber of the pressure difference balance containing the pressure spring to this conduit and in the other position connects this chamber to a return conduit to a tank.

[lite States Patent Haussler [451 Aug. 13, 1974 CONTROL DEVICE FOR LOAD-INDEPENDENT FLOW REGULATION [7 5] Inventor: Hubert Haussler, Neuheim,

Switzerland [73] Assignee: Beringer l-lydraulik R. Beringer &

C0., Neuheim, Switzerland [22] Filed: Feb. 14, 1973 [21] Appl. No.: 332,274

[30] Foreign Application Priority Data Feb. 18, 1972 Switzerland 2385/72 [52] US. Cl. l37/596.13, 60/462 [51] Int. Cl. Fl6k 13/00, F16b 1/00 [58] Field of Search 60/462; 91/414;

[56] References Cited UNITED STATES PATENTS 3,488,953 l/l970 Haussler 50/462 FOREIGN PATENTS OR APPLICATIONS Switzerland 60/462 513,340 ll/l97l Primary ExaminerEdgar W. Geoghegan Assistant Examiner-William F. Woods [57] ABSTRACT A control apparatus for load-independent flow regulation, with at least one control unit having a slide valve, a pressure difference balance, conduits leading to a pump and to a consumer and a conduit communicating the pressure in the consumer unit to the pressure difference balance, is disclosed. This last conduit has first and second sections. The first section passes through the slide valve and, when the slide valve is moved out of the neutral position, opens into an annular groove provided in the slide valve bore. The second section extends from this annular groove and has located therein an operating piston which in one position connects a chamber of the pressure difference balance containing the pressure spring to this conduit and in the other position connects this chamber to a return conduit to a tank.

8 Claims, 8 Drawing Figures PAIEmgmum arm SHEET 1 [If 4 PMENTEDAUBI 38M 3.828.818

SHEET 2 0F 4 Fig. 4

BACKGROUND OF THE INVENTION The invention relates to a control apparatus for loadindependent flow regulation. More specifically it relates to apparatus of this type'which comprises at'least one control unit with a slide valve for connecting a pump conduit with at least one conduit to a consumer unit connected to the latter, also a pressure difference balance for maintaining a constant yet adjustable pressure difference between the pump conduit and the respectively connected consumer unit conduit, as well as a conduit whichreports the pressure in the consumer unit to the pressure difference balance before the consumer unit is connected to the pump conduit.

Various embodiments of load-independent control apparatuses of this type are already known. One is described in U.S. Pat. No. 3,488,953,.another in Swiss Pat. No. 5,133,340. In both apparatuses the pressure from the consumer unit conduit is directed to the pressure difference balance through small bores, which open directly into the. bores of thecontrol slide valve and which are respectively openedorclosed by a leading edge of the respective slide valve. A disadvantage of this solution consists in' that'these small bores must' open very precisely into the slide valve bore through the relatively thick walledhousing, in'ordertobe able to be opened by the leading edge of the slide valve at the correct moment, i.e., before. the consumer unit is connected to the pump'conduit. Also, when assembling several slide valves with one. pressure difference balance in a single housingthese small'diameterchannels become too long to be accurately bored. This resulted in the control unithaving. to be divided into individual segements (so called sandwicliconstruction) inorder to be able to produce the, bores appropriately. These control segments were-then-joinedtogether by means of the rods and sealedlby gaskets between the planes of separation, which involved thedanger that the control units distorted. As a consquence, the slide valves jammed and, due to the many sealing points between the units, leakage losses with dischargeof the control fluid to the outside resulted.

The numberof these smallboreswas also increased due to the fact that a special circulation channel had to be provided for emptyingthe pressure difference balance, which channel had to pass through all slide valves in order that it couldbe immediately interrupted when actuating one of the latter. This circulation channel also had to be machined very accurately, which was only possible by the aforementioned division of the control unit into individual control segments.

It is, therefore, an object of the present invention to provide a control apparatus for lead-independent flow regulation which will overcome disadvantages described above.-

It is another object of the invention to avoid the necessity for these small bores partially by replacing them by larger bores which are easier to manufacture and partially by eliminating them by using other members and by re-arrangingthe conduits.

BRIEF SUMMARY OF THEINVENTION These objects have-been achieved by the control apparatus accordingto the invention, in which the conduit for reporting the load pressure difference balance.

' has first and second sections. The-first section passes through the slide valve and, when the slide valve is moved out of the neutral position, opens into an annular groove provided in the slide valve bore of the housing of the control apparatus. The second section extends from this annular groove in the housing and has located therein an operating piston which in one position connects the chamber of the pressure difference balance containing the pressure spring to this conduit and in the other position connects this chamber to the return conduit to a tank.

BRIEF DESCRIPTION OF THE DRAWING Embodiments of the control apparatus according to the invention are illustrated in the accompanying drawings; in which:

FIG. I shows a housing for two control slide valves and a pressure difference balance;

FIG. 2 is a section along line II-II of FIG. 1;

FIG. 3 is a section along line III-III of FIG. 1, showing the slide valve in neutral position;

FIG. 4 is a section along line IVIV of FIG. 1, showing the other slide valve in an operating position;

FIG. 5 is a vertical section through the pressure difference balance and the operating piston, along line V--V of FIG. 1;

FIG. 6 is a cross-section through the same parts, along line VIVIof FIG. 5;

FIG. 7' is a cross-section similar to FIG. 2, but through another embodiment of the control apparatus; and,

FIG. 8'is-.a section along the VIIIVIII of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The control apparatus is described hereafter with reference to one embodiment with a pressure difference balance-and two connected control slide valves. I According to FIG. 1 two control slide valves 2 and a pressure difference balance 3 are located in a housing 1'. The slide valves are mounted to move axially in bores 4-in the housing.

In' the center of the housing 1 is located a channel 5, (running horizontally in FIG. 1) which extends from the left-hand side of the housing in the figure to the pressure difference balance 3'. It is connected by way of a channel Slrunning at right-angles thereto (see also FIG. 5) to a pump which is not shown. Two return channels 6 areprovided parallel to this channel. The lower of the two channels (FIG. I) likewise terminates in the bore of the pressure difference balance 3 and is at this point connected by means of a channel 61 running at right-angles thereto to a return tank which is not shown. The upper return channel 6 is shorter, it is connected by a vertical channel 62 to the lower channel 6 and thus is likewise connected to the'return channel 61 (FIGS. 1 and 2).

Two further channels 7' parallel to the return channels 6*may be seen in FIG. 1. The latter serve for transmitting the pressure prevailing in the consumer unit to the pressure difference balance before this consumer unit is connectedto the pump. The position of both channels 7 may be seen-in FIG. 3. Similar to the channels 6, the channels 7 are connected to each other at one of their ends by a channel 71 passing at right-angles thereto, to which a further channel 72 is connected,

which leads to the pressure difference balance 3 (see also FIGS. 5 and 6).

As can be seen from FIG. 1, all these channels 5, 6, 61, 62, 7, 71 and 72 and the slide valve bores 4 are constructed as bores, which begin at one of the outer sides of the housing, The housing may therefore be machined on a boring machine, which is less expensive and quicker than on a lathe. When the manufacture is complete the channels are closed by plugs 70 at the outer side of the housing 1. Those bores which are relatively long and therefore during their manufacture tend to run out of center (deviation from a straight line) have a relatively large diameter so that they may be manufactured with a larger and therefore more rigid boring tool, whereas the shorter bores may have smaller diameters. In addition the bores are arranged with respect to each other so that they only intersect each other such that deviations from the position provided have no effect on the control functions and therefore greater tolerances can be permitted. For all these reasons the manufacture is substantially simplified and made less expensive.

The principle of constructing channels as bores is not limited solely to the aforesaid channels; further channels to be mentioned hereafter are constructed in this manner.

Between the pump channel 5 and the return channel 6, two bores 8 and 9 open into each slide valve bore 4. These bores extend at right angles to each other and lead to a consumer unit not illustrated. Depending on the position of the slide valve one of the two bores serves for the influx of control fluid to one side of the consumer unit, the other serves for the return to the tank from the other side of the latter.

Each slide valve bore 4 has at its ends a widening portion 41 of considerably greater diameter than the bore diameter itself. An annular groove 42 is located at a slight distance therefrom. These annular grooves are connected to the associated pressure indicating channels 7 by means of a connecting channel 73 respectively. The widening portions 41 are made necessary by the manufacture; as can be seen from FIG. 3 their diameter and depth are chosen such that it is possible to bore the connecting channels 73 from outside the housmg.

As can be seen from FIG. 3, the slide valve bore 4 is intersected by channels 5 and 6. The sizes of these intersections, within certain limits, is unimportant so that the channels 5 and 6 may be produced with relatively large tolerances. Certain deviations in their position with respect to the slide valve bore have no effect on the control functions.

According to FIG. 3 each control slide valve 2 has two terminal flanges 21 and two control flanges 22, 23 respectively, which are separated by a restriction 24. The control flanges 23 have leading edges 25. Constrictions 26 are provided between the control flanges 22 and the terminal flanges 21 andbetween the control flanges 23.

Each control slide valve 2 has two bores 27 respectively extending from the end of the control slide valve co-axially to its longitudinal axis to the restriction 24. At this point it is penetrated by a transverse bore 28. A transverse bore 2.9 is also present in each terminal flange 21. The latter'transverse'bores are arranged such that in the neutral position of the slide valve they are closed by the wall of the bore 4.-The longitudinal bores 27 are closed at their ends by plugs 271.

The pressure difference balance 3 (FIG. 5) is of the construction known from U.S. Pat. No. 3,488,953 and Swiss Pat. No. 513,340. A piston 31 with a restriction 32 between its two ends has a longitudinal bore 33 and a transverse bore 34, by which the pressure in the conduit 51 may be directed to the underside of the piston 31, so that the latter may be displaced against the action of a spring 35, which is located in a spring chamber 36. With respect to the already known embodiments, the present pressure difference balance 3 differs in that the pre-tensioning of this spring 35 may be adjusted by a spring washer 38 which is axially displaceable by means of a screw 37. The screw 37 extends through a cover plate so that it may be turned from outside.

The spring chamber 36 is connected to the aforementioned pressure indicating channel 72 by a short transverse channel 74 and a vertical bore 110. An operating piston 111 is arranged to slide in bore 110. Piston 111 has two flanges 112, 113. The upper edge 114 of the flange 112 is spaced from the lower edge 115 of the flange 113 by a distance which is slightly greater than the diameter of the transverse bore 74. As in the piston 31 of the pressure difference balance 3, provided between the flanges 112, 113 is a transverse bore 116, which opens into a longitudinal bore 117, which terminates at the upper side of the piston 111. Located above the piston 111 is an abutment 118, which may be displaced axially, as required, by a screw 119 in the cover plate 100.

The transverse bore 74 extends beyond the pressure difference balance 3 (FIGS. 5 and 6) and terminates in a vertical bore 120 in which an adjustable servo-excess pressure valve 121 is located. This valve corresponds to the servo-excess pressure valve according to U.S. Pat. No. 3,488,953 and Swiss Pat. No. 513,340 and is connected to a separate channel 122 which leads to the return channel 62. The valve 121 may be adjusted to a given opening pressure by a regulating screw 123 inserted in the cover plate 100.

Whereas the channel 72 is underneath the transverse channel 74, a further channel 75 passes above the latter. It opens into a channel 76 running at right-angles thereto, which intersects the common return conduit 62. Thus, a connection from the spring chamber 36 of the pressure difference balance 3 to the return channel 61 is formed.

As may be seen particularly from FIG. 6, these channels 74, 75, 76 are arranged such that they may be bored from outside, which simplifies the manufacture. The branches of these channels which are not required are then closed by plugs 70. It should also be noted that the channels only intersect each other, for example like the channels 76 and 62, so that tolerances which are as great as possible may be allowed in the manufacture.

The method of operation of this control apparatus does not differ in principle from that according to U.S. Pat. No. 3,488,953 and Swiss Pat. No. 513,340. The operations taking place therein may be seen from FIGS. 3 and 4.

If one of the slide valves 2 is moved out of the neutral position (FIG. 3), then firstly after approximately one fifth of the stroke the upper transverse bore 29 is connected to the upper annular channel 42. Due to this there occurs a continuous connection of the consumer unit connector 8 via the channels 28, 27, 29, 73, 7, 71, 72, 110 and 74 to the spring chamber 36 of the pressure difference balance 3, and this happens before the consumer unit connector 8 is connected to the pump channel 5. The function of the operating piston 111 in this connection will'be explained hereafter. Therefore, in known manner the pressure difference balance throttles the control fluid flowing out of the channel 51 and into the return channel 61 so that the pressure of the pump adjusts to the pressure of the consumer unit, increased by the increase in pressure of the force of the spring 35. The connection between the consumer unit connector 8 and the pump channel 5 via the bore 4, past the central restriction 26, is initiated by further raising the slide valve 2. However, this only takes place after approximately one quarter of the stroke, i.e., after the control flange 23 with its leading edge has been raised sufficiently far. Since the pressure in the pump channel 5 is now already higher, any unintentional return flow from the consumer unit channel '8 to the pump channel 5 is reliably avoided. The remaining three-quarters of the slide valve stroke are now available for the sensitive and progressive opening of the connection from the pump channel 5 to the consumer unit connector 8. The actuation of the slide valve may take place manually, mechanically, hydraulically, pneumatically or electro-magnetically, which will not be discussed in detail. 7

The consumer unit pressure in the channel 72 does not reach the spring chamber 36 of the pressure difference balance 3 directly, but is flrstly directed to the lower end face of the hydraulically operated piston 111. Thus the effective force presses the latter against its upper abutment 118 and opens the connection to the spring chamber 36 via the transverse bore 74, whereas the lower edge 115 of the flange 113 by lifting produces the connection between the channels 72and 74 (FIG. 5), while simultaneously the control flange 112' interrupts the connection between the channels 74 and 75. Thus the pressure may reach the spring chamber 36 of the pressure difference balance 3 and the latter, in known manner, regulates the flow from the channel 5a past the restriction 32 into the return channel 61 and thus the pressure of the portion of the control fluid flowing into the pump channel 5.

It should be noted that for this lifting a minimum distance of approximately 1 mm is sufficient, since, as aforementioned, the two leading edges 114 and 115 are separated from each other slightly more than the diameter of'the channel 74.

By means of the upper abutment screw 119, the abutment 118 may be adjusted, due to which the crosssection of the passage between the channels 72 and 74 may be adjusted. Thus, the speed of the piston 31, which moves downwards as a result of the increase in pressure, may be influenced and also the speed of the pressure rise may be adjusted.

However, the operating piston 111 has a further task, which consists in reducing the pressure prevailing in the spring chamber 36 as quickly as possible after the return of the slide valve 2 to the central position, in order that the pump is returned to a condition in which it works almost without pressure. Thus it replaces the circulation channels used in the prior art, which had a small diameter, long lengths and thus made a one-piece construction practically impossible (several slide valves in a single housing). .Since in the central position of all slide valves 2, the influx of control fluid from the consumer unit connectors 8, 9 is blocked, a slight leak from the underside of the operating piston 111 to the return 75, 76, 62 is sufficient so that the pressure in the spring chamber 36 acts on the upper surface of the operating piston 111 via the transverse bore 116 and the longitudinal bore 117 and displaces the latter downwards, whereby a connection to the return 75, 76, 62 is automatically controlled. The pressure drop in the spring chamber 36 is further increased in that, as aforesaid, the distance between the two leading edges 114, on the operating piston 111 is only slightly greater than the diameter of the transverse bore 74. Therefore with the slightest downward movement of the piston 111, the connection to the return is established.

When the slide valve 2 is actuated at the maximum cross-section of aperture limited by the stroke, between the pump channel 5 and the consumer unit connector 8, a given maximum rate of flow to the consumer unit is established, which may be influenced by the pressure difference resulting from the force of the spring 35. By means of the adjusting screw 37 in the center of the cover plate 100 within certain limits an adaptation to the pump capacity currently in service may thus be undertaken, which otherwise could only be achieved by limiting the stroke of the slide valve 2, together with a decrease in the regulating path or by conical chamfers on the leading edges 25. Due to the possibility of adjusting the spring force at the pressure difference balance, the circulating pressure, i.e., the pressure in the pump channel 5 in the central position of all the slide valves, may also be altered, so that, with an electrohydraulic anticipatory control, sufficient control pressure is available when moving from the central position.

The adjustable servo-excess pressure valve 121 in known manner limits the maximum possible pressure in the-spring chamber 36 of the pressure difference balance 3, and in the case of an excess pressure allows the control fluid flowing from the consumer unit to flow into the return channel 122 so that a further pressure increase in the pump channel 5 leads to an automatic opening of the pressure difference balance 3 to the return, whereby the latter also carries out the function of a servo-controlled excess pressure valve.

FIGS. 7 and 8 illustrate a further embodiment, offering another possibility of altering the pressure difference effective at the throttle point between the pump channel 5 and consumer unit, which throttle point may be altered by the slide valves. This embodiment is used aboveall when individual consumer units, which are actuated by the same control unit, are to receive amounts of fluid flow which are substantially smaller than the flow conveyed by the pump.

For this purpose, the common pump channel 5, which leads from the pressure difference balance 3 to the individual slide valve bores 8, 9, is provded to be somewhat more remote from the longitudinal axes of the slide valves so that it no longer has direct contact with the slide valve bores 4. This connection is firstly produced by a bore 52 located at 90 thereto, in which, however, an adjustable non-return valve is located. Amongst other things the latter has a compression spring 131, which is held by an adjusting pin 132. The latter is screwed into the housing 1 and has a slot 133 on the outer side, which serves for turning the pin 132. By altering the spring force of the non-return valve 130, the pressure drop between the pump channel 5 and the consumer unit may thus be reduced by any desired amount, so that at the throttle points which can be regulated by the slide valve 2 (leading edge 25 in FIGS. 3 and 4), a smaller pressure drop may be adjusted, due to which the rate of flow to the consumer unit is correspondingly reduced. By means of the nonreturn valve 130, with simultaneous actuation of two slide valves in one control unit, a flow from a heavily loaded consumer unit via the pump channel 5 to the slightly loaded consumer unit is avoided, which would lead to an unintentional movement of the heavily loaded consumer unit.

The advantage of the control arrangement of this invention consists above all in that the position of the bores for indicating the consumer unit pressure to the pressure difference balance 3 is such that for the first time with justifiable expense it is possible to manufacture control valves for load-independent flow regulation in a one-piece construction. This means that up to eight and more control units, which each control a consumer unit, and the pressure difference balance with an excess pressure valve may be located in a common housing. However this'does not preclude that the illustrated control apparatus may also be built as a so-called sandwich construction (individual control elements arranged together) or as a so-called mounting plate construction (allcontrol elements and the pressure difference balance arranged on a base plate, which contains all the connecting bores and connectors).

A further advantage consists in that the conduit which conveys the consumer unit pressure to the pressure difference balance, needs only to be accurately machined at certain points and in particular at points where this accuracy is readily achieved, namely in the control slide valves. Thus, without great expense, it is possible to provide a section point with a very small diameter in this conduit, which is necessary in order to be able to keep the overall length of the slide valve and of the housing as small as possible. The other sections of the conduit, namely those which are located inside the housing, may have greater diameters, which are easier to bore and with which large tolerances may be allowed .without drawbacks. Solely the annular grooves 42 must be accurately positioned, which however causes few-difficulties because they are located directly below the surface of the housing.

It is to be understood that the invention is not limited to the embodiments shown, but only by the scope of the appended claims.

I claim:

1. A control apparatus for lead-independent flow regulation, comprising at least one control unit having a housing with a bore and an annular groove communicating with said bore, and a slide valve in said bore movable to and from a neutral position, a first conduit connecting said slide valve to a pump, a second conduit connecting said slide valve to a consumer unit, a pressure difference balance for maintaining a constant, but adjustable pressure difference between said first conduit" and the respectively connected second conduit,

said pressure difference balance having a chamber, a pressure spring in said chamber, a third conduit for communicating the pressure in the second conduit to the pressure difference balance prior to connection of the latter to said first conduit, said third conduit having a first section which is located in said slide valve and which, when said slide valve is displaced from the neutral position, opens into said annular groove, and also having a second section which extends from said annular groove, an operating piston which is located in said second section and which in one position connects said chamber of said pressure difference balance to said third conduit and in another position connects said chamber to a reservoir.

2. A control apparatus according to claim 1, wherein said first section of said third conduit comprises a first transverse bore connected to said second conduit, a longitudinal bore connected to said first transverse bore, and a second transverse bore penetrating said longitudinal bore, said second transverse bore opening into said annular groove upon actuation of said slide valve.

3. A control apparatus according to claim 1, wherein said second section comprises a bore, said operating piston being arranged to be movable in said lastmentioned bore, said last-mentioned bore having a first connecting channel leading to said pressure difference balance, and a second connecting channel leading to the reservoir.

4. A control apparatus according to claim 3, wherein said operating piston has two control flanges, one of said flanges adapted to block the connection between said second section and said first connecting channel, and the other flange being adapted to block the connection between said first connecting channel and the reservoir.

5. A control apparatus according to claim 3, including an abutment in said last-mentioned bore for limiting the movement of said operating piston, and an adjusting screw for axially adjusting the position of said abutment.

6. A control apparatus according to claim 4, said operating piston having edges spaced apart by a distance which is slightly greater than the diameter of said first connecting conduit between said last-mentioned bore and said pressure difference balance.

7. A control apparatus according to claim 1, wherein said second section comprises a third connecting channel extending at an angle from said annular groove and wherein said bore has a widening portion at each end, the diameter and depth of said widening portion being such that an extension of said third connecting channel lies inside said widening portion.

8. A control apparatus according to claim 5, wherein said operating piston has a transverse bore connected to said chamber, and a longitudinal bore connected to said transverse bore, said longitudinal bore terminating at that end face of said operating piston which is adjacent said abutment.

1: Eon-ON maze-m Moo '4 R92 R1 2 Dated Invenmfle) It 113 certified that error appears in the above-identified patent and that said Letters Patentere hereby corrected as shown below:

Column 7 line 51, change "lead-independent" to -load'- independent Signed arid sealed this 5th'day"o:f' November 1974..

' (SEAL) Attest:

McCOY M; GIBSON JR. c. MARSHALL DANN Attes'ting Officer Commissioner of Patents FORM 90-1050 (IO-69) USCQMM-DC GONG-P09 0 u. I. oovlmmlm' nmtmo omcl nu 0-860-"4. 

1. A control apparatus for lead-independent flow regulation, comprising at least one control unit having a housing with a bore and an annular groove communicating with said bore, and a slide valve in said bore movable to and from a neutral position, a first conduit connecting said slide valve to a pump, a second conduit connecting said slide valve to a consumer unit, a pressure difference balance for maintaining a constant, but adjustable pressure difference between said first conduit and the respectively connected second conduit, said pressure difference balance having a chamber, a pressure spring in said chamber, a third conduit for communicating the pressure in the second conduit to the pressure difference balance prior to connection of the latter to said first conduit, said third conduit having a first section which is located in said slide valve and which, when said slide valve is displaced from the neutral position, opens into said annular groove, and also having a second section which extends from said annular groove, an operating piston which is located in said second section and which in one position connects said chamber of said pressure difference balance to said third conduit and in another position connects said chamber to a reservoir.
 2. A control apparatus according to claim 1, wherein said first section of said third conduit comprises a first transverse bore connected to said second conduit, a longitudinal bore connected to said first transverse bore, and a second transverse bore penetrating said longitudinal bore, said second transverse bore opening into said annular groove upon actuation of said slide valve.
 3. A control apparatus according to claim 1, wherein said second section comprises a bore, said operating piston being arranged to be movable in said last-mentioned bore, said last-mentioned bore having a first connecting channel leading to said pressure difference balance, and a second connecting channel leading to the reservoir.
 4. A control apparatus according to claim 3, wherein said operating piston has two control flanges, one of said flanges adapted to block the connection between said second section and said first connecting channel, and the other flange being adapted to block the connection between said first connecting channel and the reservoir.
 5. A control apparatus according to claim 3, including an abutment in said last-mentioned bore for limiting the movement of said operating piston, and an adjusting screw for axially adjusting the position of said abutment.
 6. A control apparatus according to claim 4, said operating piston having edges spaced apart by a distance which is slightly greater than the diameter of said first connecting conduit between said last-mentioned bore and said pressure difference balance.
 7. A control apparatus according to claim 1, wherein said second section comprises a third connecting channel extending at an angle from said annular groove and wherein said bore has a widening portion at each end, the diameter and depth of said widening portion being such that an extension of said third connecting channel lies inside said widening portion.
 8. A control apparatus according to claim 5, wherein said operating piston has a transverse bore connected to said chamber, and a longitudinal bore connected to said transverse bore, said longitudinal bore terminating at that end face of said operating piston which is adjacent said abutment. 